The family of a man shot and killed by Concord police in Antioch last year has filed a federal civil rights lawsuit, alleging he was the victim of excessive force.

On May 10, Concord police were conducting surveillance on the Barcelona Circle home of Charles Burns, 21, who was being sought on a warrant for alleged drug sales. Burns exited the home and got into a truck driven by his friend Bobby Lawrence, police officials said.

As undercover officers tried to stop the truck, officials said, Lawrence sped up and rammed an occupied police car. Burns then ran from the truck and was shot dead by two officers who reported that they had feared for their safety, police said.

But in a lawsuit filed Tuesday in U.S. District Court in San Francisco, John and Tammie Burns said their son and Lawrence hadn't realized that the men who accosted them at gunpoint were police officers in undercover vehicles.

The suit said the two men were heading to Walmart to buy a Mother's Day card. Lawrence kept on driving despite being rammed by one of the police vehicles because he was "trying to escape this onslaught of unknown assailants," said the suit, which also names Lawrence as a plaintiff.

Burns was unarmed and "took no aggressive action but rather yielded to the officers and cowered his shoulders" moments before officers "lined up in firing squad execution fashion and unloaded their weapons on the defenseless Burns with full intent to execute him," said the suit filed by attorney Peter Johnson.

The suit seeks unspecified damages and names the city of Concord and Police Chief Guy Swanger as well as Contra Costa County and the district attorney's office.

The defendants have not responded to the suit in court. Concord City Attorney Mark Coon said he could not comment on pending litigation.